The invention relates to an optical signal modulation method and an optical signal transmission system for the high-speed transmission systems such as Time Division Multiplexing (TDM) or Dense Wavelength Division Multiplexing (DWDM).
The introduction of optical fiber amplifier technology in EDFA (Erbium-doped Fiber Amplifier) overcomes the problem of optical power loss during transmission in a TDM/DWDM system and increase significantly the permitted transmission distance. However, the inherent noise in EDFA degrades the Signal-to-Noise Ratio (SNR) of the system and limits the further extension of passive relay transmission distance. Although the value of SNR can be increased with the strengthening of power input in a single channel, higher power input will contribute to non-linear effect and distortion of pulse form in the optical signal due to chromatic dispersion effect, which will degrade significantly the quality of optical signal transmission. For a DWDM system, the non-linear effect and chromatic dispersion effect will influence much more the optical signal transmission because multi-channel information carried on different wavelengths transmits in an optical fiber.
In order to overcome the Amplified Stimulated Emitting (ASE) noise of EDFA and non-linear effect by high power input and to extend the distance of passive relay transmission, many technologies (Forward Error Correction (FEC), RAMAN Amplification, etc.) are employed in optical signal transmission systems, such as a DWDM system. And commercial systems supporting above 4000 km for passive relay distance are available now. However, the systems are usually used in submarine environments due to its strict requirements for ambient environment and technologies as well as high cost resulting from RAMAN Amplification. And it isn""t practical in common optical signal transmission systems.
The object of the present invention is to provide an optical signal modulation method that can depress effectively the non-linear effect in the transmission of optical signal to extend the distance of passive relay transmission in optical signal transmission systems, and long distance passive relay optical fiber transmission systems with low cost is thus available. Such an optical signal modulation method comprises of the following procedures:
a. A continuous waveform laser generates optical signals, and a high-speed data signal driver generates a high-speed data stream;
b. The optical signals and high-speed data stream are input to an intensity modulator synchronously, where the optical signals are modulated into optical intensity pulses according to the high-speed data stream;
c. The optical intensity pluses modulated and the high-speed data stream are input to a phase modulator synchronously, where the optical intensity pulses are modulated in phase into high-speed optical signals with chattering according to the high-speed data stream;
d. The high-speed optical signals are input to the optical fibers to transmit.
Another object of the present invention is to provide an optical signal transmission system implementing said optical signal modulation method, said system comprises:
An optical signal-emitting module;
Optical fiber links to amplify and transmit high-speed optical signals;
An optical signal-receiving module;
Wherein the optical signal-emitting module comprises of: A continuous waveform laser to generate optical signals;
An intensity modulator to modulate the intensity of optical signals into optical intensity pulses;
A phase modulator to modulate the phases of optical intensity pulses into high-speed optical signals with chattering;
A high-speed data signal driver to generate high-speed data stream controlling intensity modulation and phase modulation.
Said optical fiber links comprise:
Transmitting optical fibers to transmit high-speed optical signals;
Two optical fiber amplifiers to amplify high-speed optical signals and implement full compensation to optical power;
A chromatic dispersion compensator to compensate the chromatic dispersion of high-speed optical signals;
Wherein, the chromatic dispersion compensator is series-wound between said two optical fiber amplifiers, which connect with said transmitting optical fiber and said optical signal-receiving module respectively at the other end.
According to the invention, the phase modulator added in the optical signal-emitting module of the optical signal transmission system modulates the phases of optical signals after intensity modulation so as to realize appropriate chattering modulation to optical intensity pulses. The phase modulation process will not alter the waveform of the optical signal pulses in Time Division, but it will actually alter the spectrum distribution, which will result in alteration in chromatic dispersion effect, non-linear effect, and interaction between them in the transmission of optical signal pulses. Therefore, appropriate chattering modulation can depress effectively the non-linear effect in the transmission of optical signal pulses through the interaction between chromatic dispersion and non-linear effects, which increases greatly the power input of individual channels and extends the passive relay regeneration distance of the system.